This invention relates to television and is directed to method and apparatus for improving the sharpness of reporduced television pictures. This invention is applicable to monochrome and color television, and in the latter case, provides enhancement of the sharpness of both luminance and chroma channels.
The reproduction of geometrical details is an important problem in television. Many factors are involved, but when the number of lines per frame and of frames per second has been fixed, the horizontal resolution is largely a function of system bandwidth. With the presently used National Television System Committee or NTSC color television system, it is generally admitted that a 4.2 MHz luminance bandwidth, leading to a 120 ns transition duration is acceptable to provide a horizontal resolution which matches in the correct proportion the vertical resolution which is imposed by the 525 lines per frame standard. In the color difference signal channels, a 1.3 MHz bandwidth for the I channel and a 500 KHz bandwidth for the Q channels are considered as acceptable.
It is a recognized fact that many links in the chain of television processing are often incapable of reaching the foregoing bandwidth requirements: video cameras, transmitting channels of reduced bandwidth, video tape recorders, etc.-- often are not capable of full resolution. More particularly, the recent popularity of helical scan video recorders, due to their high portability and low cost, has resulted in the appearance of low luminance (2 MHz) and chroma (350 KHz) bandwidth pictures which are even present, sometimes, in sophisticated broadcasting environments. These low bandwidth pictures, generated because of the convenience of the equipment, are immediately recognizable by their "blurriness" and lack of resolution. The video bandwidth then has to be increased, if an increase of sharpness is to be witnessed. However, this is not always practical in television systems. For example, in a color camera, any bandwidth increase leads to tougher requirements for registration, quality of the pick-up tube, better optics etc.--and to a significant cost increase. For transmission systems, the transmission cost is directly proportional to the bandwidth. In the case of video tape recorders, higher bandwidth requires higher head-to-tape writing speed and generally more delicate and costlier mechanical structures.
For all these reasons, many attempts have been made in the past to increase the apparent sharpness of a television picture without increasing bandwidth capabilities throughout the system.
A first type of device known to the art for performing to some extent these functions were image enhancers and aperature equalization systems. These systems are described in the following publications: "Horizontal Aperature Equalization" by A. N. Thiele, Radio and Electronic Engineer, Vol. 40, No. 4, October 1970, page 193; CBS Laboratories Mark IV Automatic Image Enhancer Technical Bulletin, March 1974; Philips Colour Telecine System brochure, page 25 to 27.
Those devices and systems generally operated in a strictly linear fashion. A high frequency signal was derived from the input transition and was added to it without envelope delay error and with the proper phasing. In those conditions, the output transition was apparently shorter in duration than the input transition. However those devices had two apparent drawbacks:
First, noise, as well as signal, was enhanced in the process. In order to avoid this effect, some of those devices had a threshold in the enhancement path, threshold which, naturally would defeat enhancement for low-contrast television details.
Second, sharpening could not be obtained without preshoot and overshoot. While actual duration of a transition did not vary; the passage from black to white did not happen in the same fashion. The visible transition zone on the screen was as wide as before, but objects seemed to be surrounded with black and white margins. Thus, the preshoot and overshoot gave to the picture an unnatural or "cartoon" look.
A second type of device was found in video "crispeners." A "crispener" is essentially different in its nature, and has more ambitious goals, than an enhancer. A crispener is a transition-processing device whose purpose is the generation by non-linear processing of frequencies higher than the upper limit of the bandwidth of the input signal. Its objective is to shorten a transition without introducing preshoot and overshoot. Examples of prior art crispeners are described in U.S. Pat. No. 2,740,071 to Goldmark and Reeves, U.S. Pat. No. 2,851,522 to Hollywood, and in the article entitled "A New Technique for Improving the Sharpness of Television Pictures," Goldmark and Hollywood, Proceedings of the IRE, October 1951, page 1314.
Those two patents and the article described various approaches to increase sharpness which were based upon the following steps:
First, the video signal was submitted to a first differentiation,
Next, the differential waveform duration was reduced by means of a threshold in U.S. Pat. No. 2,740,071 and in U.S. Pat. No. 2,851,522 by means of a rectifier followed by a reactance applied to generate spikes of shorter duration than the input signal transition.
Those circuits were used in conjunction with the CBS field sequential color television system in 1951 and were abandoned when the CBS system was dropped as a color standard in 1955. Those circuits did not reach the market place later on (nor did circuits of similar nature) because of their weaknesses:
1. Those circuits were costly and complicated.
2. Those approaches were transition sign dependent and required two paths of rectification.
3. The correction waveform was essentially asymmetrical and an overshoot was unavoidable.
4. Short-line details, such as sine-square pulses, were reduced in contrast, as compared with true high bandwidth, linearily processed signals (see e.g. Proceedings of the IRE, October 1951, page 1318, Fig. 14).
5. Tracking, that is generation of a correction waveform strictly proportional to the transition level over variations as much as 40db, was impossible. As a result of those circuits large transitions exhibited overshoots, while small transitions were undercrispened.
Thus, the picture improvement brought about by those techniques, although noticeable, was not high enough to justify, in most applications, the high costs of circuit implementation and the inherent risks of instability necessarily resulting from complexity.